This invention relates to a process for vapor phase polymerization of monomers in a vapor-phase, quench-cooled reactor vessel in which off-gas from the reactor vessel is recycled. More specifically, this invention relates to a method and apparatus for recovering and recycling entrained solids (sometimes referred to as "fines") which are undersized particles of polymer product and solid catalyst particles from the off-gas from the reactor vessel.
Vapor phase polymerization processes using vapor-phase, quench-cooled reactor vessels have been described in U.S. Pat. Nos. including 3,652,527; 3,957,448; 3,965,083; 3,970,611; 3,971,768; 4,003,712; 4,101,289; 4,129,701; 4,130,699; 4,337,069; and 4,372,919 and Reissue Pat. No. 30,148, all incorporated by reference herein. These references describe polymerization processes and apparatuses in which polymer is formed from gaseous monomer in horizontal/stirred bed, vertical/stirred bed or fluidized bed reactor vessels. Typically, in operation of such processes and apparatuses, particles of polymer are formed around solid catalyst particles. Typically, polymer power particles are formed in a particle size distribution mirroring the distribution of the solid catalyst particles used in the process. Thus, depending upon the particle size distribution of solid catalyst particles, more or fewer small polymer particles are formed. Very small polymer particles (typically about 5 to 600 microns in average diameter) cause problems in the overall polymerization process by being entrained in various parts of monomer or recycle gas transfer lines. Such small polymer particles tend to react in the condenser/scrubber system of the process in which off-gas is partially condensed to a liquid which is recycled to the reactor as quench liquid, resulting in high molecular weight particles which form a hard sludge-like buildup which can clog various pipes, conduits, vessels, outlets and nozzles of the apparatus used in the process. As a result, the polymerization process must be shut down for cleaning.
Another problem associated with entrained solids in the recycle quench liquid is that these particles have a higher molecular weight than polymer normally formed in the reactor. This is due to a low concentration of molecular weight control agent such as hydrogen in the condensed recycle liquid where such entrained polymer particles are forming. When these high molecular weight particles are recycled to the main reactor and are mixed with normal polymer product, such high molecular weight (low melt flow) polymer contaminates the normal product polymer exiting the reactor vessel and appear as specks in polymer films made from such mixed polymer product.
If the entrained solids are removed by screening various transfer lines, a substantial amount of polymer (0.1 to 5 wt. % of total polymer production) is wasted.
This invention describes a method and apparatus to remove entrained solids from the reactor off-gas without permitting substantial continued polymerization of such fines, and recycling the fines directly to the reactor vessel.
Although the art describes polymerization apparatus and processes generally useful for this invention, including U.S. Pat. No. 4,372,919 which discloses a horizontal bed paddle wheel agitator, vapor phase polymerization process in which fines are separated from an off-gas by a cyclone, this invention is directed to a method and apparatus by which fines can be recycled directly to a reactor without undergoing undesirable continued polymerization outside the reactor vessel in the presence of a substantially different hydrogen concentration than in the reactor. It has been found in practice that return of polymer fines directly to a reactor from a cyclone typically is not feasible because there is insufficient motive force to transport the fines particles. This invention provides a means to transport fines directly into a reactor.